Melamine pyrophosphate is widely used as a flame retardant additive in paints, coatings, and extruded plastics such as polyamides, polyesters, and polyolefins. In extruded plastics, the stability of the melamine pyrophosphate is important. Decomposition of contaminants, such as melamine orthophosphate, at extrusion temperatures, leads to the evolution of gases and vapors causing foaming and other effects adversely affecting the quality of the extruded product and the efficiency of the flame retardant properties. The thermal stability of melamine pyrophosphate samples is conveniently measured by thermogravimetric analysis (TGA). TGA analyses typically show the weight % loss of the test sample versus temperature as the temperature is increased at a constant rate, e.g., 10.degree.C/min. Additionally, the derivatives of the TGA curve may be plotted, showing the rate of weight loss as weight % loss/min.
Melamine pyrophosphate is conventionally prepared by heating an aqueous slurry of 2 moles of melamine in 4 moles of hydrochloric acid at 80.degree.C to 90.degree.C, forming a solution of melamine dihydrochloride. One mole of tetrasodium pyrophosphate is added, the temperature maintained for about 30 minutes, and the slurry quenched. The precipitated melamine pyrophosphate must be thoroughly washed to remove the by-product sodium chloride and dried. The process is described by Fessler in U.S. Pat. No. 3,914,193. However, the pyrophosphate ion is reported (Corbridge, D.E.C., Structural Chemistry of Phosphorus, Elsevier Scientific Publishing Co., New York, N.Y., 1974, p. 128) to decompose to orthophosphate at low pH and at elevated temperature, i.e., under the conditions necessary for the reaction between melamine dihydrochloride and tetrasodium pyrophosphate. At polymer extrusion temperatures that can exceed 300.degree.C, melamine orthophosphate is less stable than melamine pyrophosphate. The presence of residual sodium and chloride ions also reduces the stability of melamine pyrophosphate under melt extrusion conditions.
Melamine pyrophosphate when prepared according to the methods of the prior art, has borderline stability when used in higher temperature melt extrusion, for instance in polyamide extrusions, due to impurities such as melamine orthophosphate. Efforts to minimize "hot spots", where decomposition of the melamine pyrophosphate flame retardant can occur in the extruder, have included methods to improve the dispersability of melamine pyrophosphate.
Muehlemann and Schmid, in U.S. Pat. No. 4,088,752, and subsequently in U.S. Pat. No. 4,122,162, disclose the preparation of various organic basesalts of oxo-acids of phosphorus and mixed salts of oxo-acids of phosphorus and hydrofluoric acid by passing a salt of the oxo acid through a cation exchange resin and mixing the effluent with an alcoholic solution of an amine, such as an ethoxylated amine, or an alcoholic solution of the amine monohydrofluoride. The salts are isolated by evaporation of the solvent or by the addition of another solvent such as acetone. The technique is not applicable to the preparation of melamine condensed phosphates, and the use of organic solvents is undesirable.
Watanabe, in Japanese Kokai Patent Hei-8-48508 prepared guanidine cyclo-6-phosphate (guanidine hexametaphosphate) by passing a solution of lithium cyclo-6-phosphate through a cation exchange resin and adding to the cooled resulting solution a solution of guanidine phosphate or guanidine carbonate. The product precipitated on standing and was isolated by filtration and washing with acetone. While improving the purity of the cyclo-6-phosphate, Watanabe uses a solution of the organic base, corresponding to the use of melamine hydrochloride in Fessler's process above.
It is desirable to improve the thermal stability and purity of condensed phosphates in general, and for melamine pyrophosphate for use as a flame retardant in the melt processing of polymers in particular, and to improve the manufacturing process. The present invention provides such a process and melamine pyrophosphate with improved stability and enhanced flame retardant properties.